Since traction type CVT for automobiles has a large torque transmission capacity and is used under severe conditions, it is essential from the standpoint of the power transmission that the traction coefficient of a traction oil used therefor must be sufficiently greater than the minimum value in the temperature range in which the oil is used, namely the traction coefficient at a high temperature (120° C.) must be sufficiently greater than the value prescribed in the design of CVT.
Since the traction oil also assumes a role as an ordinary lubricant oil in CVT, it is necessary that the traction oil has a high viscosity sufficient to maintain an oil film even at a high temperature in order to prevent frictional abrasion. On the other hand, the traction oil must have a low viscosity even at a low temperature (low-temperature fluidity) in order to provide low temperature startability in cold areas such as in northern America and northern Europe. Stated otherwise, the temperature dependency of the viscosity must be small, namely the viscosity index must be high.
In view of the foregoing background, the present inventors developed a base oil compound for a high performance traction oil exhibiting such high high-temperature traction coefficient, high viscosity index and excellent low-temperature fluidity which were not achieved before (see Patent document 1).
However, a design of CVT requires that the high-temperature traction coefficient, low-temperature fluidity and viscosity index must be satisfied at a still higher level.
Patent document 1: Japanese Patent Application Laid-Open No. 17280/2000